def prost(x):
    k = 0
    for d in range(1, x+1):
        if x%d==0:
            k+=1
    if k==2:
                return True
    else:
                return False
maxx = 0
num=int(input())
if num >= 1000 and num <= 9999:
    n1= str(num)
    for cifa in range(len(n1)):
        if prost(int(n1[cifa]))==True:
            maxx = max(maxx,int(n1[cifa]))
    for chislo in range(len(n1)-1):
        if prost(int(n1[chislo]+n1[chislo + 1])) == True:
            maxx = max(maxx,int(n1[chislo] + n1[chislo + 1]))
    for chislo in range(len(n1)-2):
        if prost(int(n1[chislo]+n1[chislo + 1] +n1[chislo + 2])) == True:
            maxx = max(maxx,int(n1[chislo] + n1[chislo + 1] +n1[chislo + 2]))
    for chislo in range(len(n1)-3):
        if prost(int(n1[chislo]+n1[chislo + 1] +n1[chislo + 3])) == True:
            maxx = max(maxx,int(n1[chislo] + n1[chislo + 1] +n1[chislo + 2] + n1[chislo+3]))

    print(maxx)
else:
    print(0)
    

                
